The present invention relates to a fault location method of detecting a fault point on a cable in which a ground fault occurs by means of a pulse reflection method.
When a ground fault or the like occurs on a cable, a fault section is disconnected by a protection relay apparatus and after the fault point has been detected exactly, fault recovery work is performed at the fault location. A pulse reflection method is known as one of methods of detecting the fault point location. In the pulse reflection method, an electrical pulse, for the purpose of measuring a distance to a fault point location, is applied to the cable from a measuring end of the cable and a reflected wave generated by an electric discharge at the fault point is received back at the measuring end, so that a distance from the measuring end to the fault point can be calculated.
FIG. 4 is a diagram illustrating the principle of the well known and widely used pulse reflection method.
FIG. 4, a square transmission pulse 1 applied to the measuring end of the cable reaches the fault point after a time .tau.. In this case, a time until a reflected wave 2 is observed after the transmission wave has been observed is 2.tau..
When a cable propagation velocity of an electrical pulse is S, and a distance from the measuring end to the fault point is L, the following equation is given: EQU 2.tau.=2L/S (1)
A distance L to the fault point can be calculated by using the equation (1).
FIG. 5 shows a waveform of the propagation wave 1 which propagates in the cable.
As shown in FIG. 5, the propagation wave 1 propagating in the cable has a smooth, gradually rising portion and a smooth gradually falling portion.
A break hole, in the form of a pinhole, is typically formed in an insulating layer of the cable at the fault point so that a portion between a conductor and a shield layer is air-insulated by the break hole. Accordingly, when the electrical pulse, having a predetermined voltage, is applied from the measuring end of the cable to generate the electric discharge at the fault point and the reflected wave from the fault point is received, a phenomenon as shown in FIG. 6 occurs.
FIG. 6 is a graph showing a relation of the propagation wave 1 which has reached the fault point and an electric discharge starting voltage Vb at the fault point. In the graph of FIG. 6, the abscissa represents a time and the ordinate represents a voltage.
As shown in FIG. 6, after the propagation wave 1 has reached the fault point, its voltage is increased at a constant rate and reaches the electric discharge starting voltage Vb after a time .DELTA.T has elapsed. When the propagation wave 1 reaches the discharge starting voltage Vb, the air insulation in the break hole is broken and the electric discharge starts, so that the reflected wave is generated. Accordingly, when an actually measured value of the time until the reflected wave is received after the electrical pulse has been applied at the measuring point is t and a real turnaround propagation time of the electrical pulse is 2.tau., the actually measured value t is expressed as follows: EQU t=2.tau.+.DELTA.T (2)
As represented by the equation (2), the actually measured value t includes a discharge delay time .DELTA.T produced as an error in accordance with an actual voltage increasing speed of the propagation wave 1. For example, the propagation velocity of the electrical pulse in the cable is about 160 to 200 m per .mu. second. Accordingly, when the discharge delay time .DELTA.T is assumed to be 0.5 .mu. second, the detection error is about 80 to 100 m. Thus, the conventional pulse reflection method must be used for detection of the fault point on condition that the detection error to this extent is contained in the measured value.
However, if the fault point is detected correctly, the fault recovery work can be made rapidly. Particularly, in an underground cable which can not be inspected directly with the naked eye, the detection of the fault point with higher accuracy is desired.